1. Field of the Invention
The present invention relates to a demodulator circuit for a television multi-channel sound signal (referred to as TV multi-channel sound signal hereinafter) and more particularly, to a TV multi-channel sound signal demodulator circuit in which a noise component included in a sub-channel signal constituting a TV multi-channel sound signal can be removed.
2. Description of the Prior Art
According to a TV multi-channel sound broadcasting system currently employed in the U.S., a TV multi-channel sound signal comprises a main channel signal, a stereo pilot signal, a sub-channel signal, a second sound signal and the like, which are frequency-multiplexed. More specifically, in the case of, for example, stereophonic broadcasting, the main channel signal comprises a stereo sum (L+R) signal of the audio band (50 Hz to 15 KHz). The above described stereo pilot signal is set to have a frequency equal to the frequency f.sub.H (15.734 KHz) of a horizontal synchronizing signal of a TV signal. In addition, the above described sub-channel signal comprises a signal obtained by DSB (double sideband) modulation of a sub-carrier having a frequency of 2f.sub.H (31.468 KHz) by a stereo difference (L-R) signal (50 Hz to 15 KHz). A sound carrier of the TV broadcasting signal is frequency-modulated by the TV multi-channel sound signal obtained by frequency-multiplexing the main channel signal, the stereo pilot signal, the sub-channel signal and the like, to be transmitted.
On the other hand, on the receiving side of the TV signal, the TV multi-channel sound signal is extracted from the frequency-modulated sound carrier as received by an IF (intermediate frequency) detector circuit in a TV receiver. The stereo pilot signal of frequency f.sub.H in the extracted TV multi-channel sound signal is applied to a PLL (phase locked loop) circuit provided in a demodulator circuit in the TV receiver, so that the sub-carrier of frequency 2f.sub.H is reproduced. The stereo difference (L-R) signal is demodulated from the sub-channel signal in the above described extracted TV multi-channel sound signal using this reproduced sub-carrier. Finally, the demodulated stereo difference (L-R) signal and the stereo sum (L+R) signal of the main channel signal in the above described extracted TV multi-channel sound signal are matrixed, so that right and left stereo signals R and L are reproduced.
The above described demodulator circuit for the TV multi-channel sound signal in the U.S. is disclosed in, for example, an article by H. Fukaya et al., entitled "Decoder System ICs for US Multichannel TV Sound", NEC TECHNICAL JOURNAL, Japanese publication issued on Mar. 5, 1986, Vol. 39, No. 3, pp. 4-7.
In general, in the TV receiver, a sound signal is obtained by extracting an FM signal from a received TV signal and FM-detecting the same. As a method for extracting such a sound FM signal, generally, a method for extracting a sound FM signal from a video detection output from a video detector circuit, i.e., an inter carrier system or a method for separately detecting a video signal and a sound signal, i.e., a split carrier system can be used. More specifically, in the split carrier system, since a video and a sound are separated and independently detected, there is little effect of the video signal on the sound signal after detection. On the other hand, in the inter carrier system, since a beat signal of 4.5 MHz, i.e., the frequency corresponding to the difference between frequencies of a video carrier and a sound carrier produced in the detection output when the video signal is detected, is used as a sound FM signal, the obtained sound FM signal may be subjected to abnormal modulation by the video signal due to interference between the video signal and the sound signal.
Particularly, in the TV multi-channel sound broadcasting system in the U.S., since frequencies of the stereo pilot signal and the sub-channel sub-carrier are selected to be f.sub.H and 2f.sub.H, respectively, as described above, interference with respect to the same channel occurs between a horizontal synchronizing signal in the received video signal and the stereo pilot signal and between a second harmonic (31.5 KHz) of the horizontal synchronizing signal and the sub-carrier, so that grating noise referred to as a so-called buzz sound is produced in the reproduced sound output. The principle of generation of such a buzz sound is described in, for example, an article by T. Murakami et al., entitled "Buzz-Beat Interference in Multi-Sound Television Receivers", Japanese publication "Television", Vol. 26, No. 6, 1972, pp. 459-467.
More specifically, in the above described multi-channel sound signal demodulator circuit in which the sub-carrier of 2f.sub.H is reproduced in response to the stereo pilot signal of f.sub.H by the PLL circuit and the stereo difference (L-R) signal is demodulated from the sub-channel signal using this sub-carrier, the stereo pilot signal of f.sub.H in the sound signal is subjected to abnormal modulation by the horizontal synchronizing signal of f.sub.H in the video signal due to the above described interference in the detector circuit.
FIG. 1 is a waveform diagram for explaining modulation of the stereo pilot signal of f.sub.H by the horizontal synchronizing signal of f.sub.H. More specifically, FIG. 1(a) schematically illustrates a synchronizing signal including a horizontal synchronizing signal and a vertical synchronizing signal, FIG. 1(b) shows an original stereo pilot signal of f.sub.H and FIG. 1(c) shows a stereo pilot signal in which phase distortion is caused due to the effect of the horizontal synchronizing signal mixed in the sound signal in the detector circuit.
As a result of such phase distortion of the stereo pilot signal, a phase shift occurs in the PLL circuit. Thus, the sub-carrier of 2f.sub.H out of phase is applied to a sub-channel demodulator circuit. The demodulation level of the stereo difference (L-R) signal output from the sub-channel demodulator circuit changes depending on the above described phase shift. If the above described interference between the horizontal synchronizing signal and the stereo pilot signal always occurs, the phase shift of the sub-carrier is constant. Thus, the demodulation efficiency of the stereo difference (L-R) signal is also constant, resulting in no problem. However, the synchronizing signal in the TV signal includes a vertical synchronizing signal period (referred to as vertical period hereinafter) with a period of 1/60 seconds. There exists no horizontal synchronizing signal in the vertical period. Consequently, the stereo pilot signal is not affected by the horizontal synchronizing signal during the vertical period as shown in FIG. 1(c). Thus, the demodulation efficiency of the stereo difference (L-R) signal would differ in the horizontal period and the vertical period of the video signal. Considering this phenomenon from the reverse viewpoint, it seems that the generated sound signal is affected by the video signal only in the vertical period of the video signal.
FIG. 2 is a waveform diagram for explaining the above described phenomenon, where FIG. 2(a) schematically shows the situation where a stereo pilot signal of frequency f.sub.H applied to a PLL circuit is affected by the horizontal synchronizing signal of f.sub.H, and FIG. 2(b) shows a demodulated stereo difference (L-R) signal output from a sub-channel demodulator circuit. In FIG. 2, the period A represents a vertical period and the period B represents a horizontal period. As shown in the above described FIG. 1(c) and FIG. 2(a), since in the vertical period A, the stereo pilot signal (a) is not affected by a horizontal synchronizing signal, the obtained stereo difference (L-R) signal (b) has a constant demodulation level. On the other hand, since in the horizontal period B, the pilot signal (a) is affected by the horizontal synchronizing signal, the demodulation level of the stereo difference (L-R) signal (b) changes. More specifically, since sub-carriers having different phases are applied to the sub-channel demodulator circuit with a frequency of 60 Hz, the demodulation level of the stereo difference (L-R) signal changes with the frequency of 60 Hz as shown in FIG. 2(b). More specifically, the portion corresponding to the period A of the stereo difference (L-R) signal as shown in FIG. 2(b) acts as a 60 Hz noise component. The stereo difference (L-R) signal is applied to a matrix circuit, to be used for reproducing right and left stereo signals R and L. However, since the stereo difference (L-R) signal includes the 60 Hz noise component as described above, 60 Hz noise is produced in the stereo signals R and L which are demodulation outputs of the matrix circuit.
Additionally, as described above, interference occurs between a second harmonic of a separately mixed horizontal synchronizing signal and a sub-carrier of 2f.sub.H. More specifically, due to such a second harmonic, the sub-carrier output from the PLL circuit is subjected to abnormal modulation. As a result the demodulated stereo difference (L-R) signal is subjected to a change in amplitude. Due to such a change in amplitude, the stereo difference (L-R) signal includes the 60 Hz noise component. In particular, the larger the amplitude of the horizontal synchronizing signal in the video signal is, the more easily the demodulation output is affected by phase distortion during the horizontal period, so that 60 Hz noise becomes significant. More specifically, when a luminance signal level, i.e., a white level of the video signal is high, the 60 Hz noise occurs conspicuously.
As described in the foregoing, in the conventional TV multi-channel sound signal demodulator circuit, 60 Hz noise is produced in the stereo difference (L-R) signal. Consequently, the 60 Hz noise and noise of the harmonic are mixed in the right and left stereo signals obtained by matrixing, so that a grating buzz sound is produced in the sound signal.